// Copyright (C) 2013-2021 Internet Systems Consortium, Inc. ("ISC")
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

#include <config.h>

#include <d2srv/d2_log.h>
#include <d2srv/dns_client.h>
#include <dns/messagerenderer.h>
#include <stats/stats_mgr.h>
#include <limits>

namespace isc {
namespace d2 {

namespace {

// OutputBuffer objects are pre-allocated before data is written to them.
// This is a default number of bytes for the buffers we create within
// DNSClient class.
const size_t DEFAULT_BUFFER_SIZE = 128;

}

using namespace isc::util;
using namespace isc::asiolink;
using namespace isc::asiodns;
using namespace isc::dns;
using namespace isc::stats;

// This class provides the implementation for the DNSClient. This allows for
// the separation of the DNSClient interface from the implementation details.
// Currently, implementation uses IOFetch object to handle asynchronous
// communication with the DNS. This design may be revisited in the future. If
// implementation is changed, the DNSClient API will remain unchanged thanks
// to this separation.
class DNSClientImpl : public asiodns::IOFetch::Callback {
public:
    /// @brief A buffer holding response from a DNS.
    util::OutputBufferPtr in_buf_;

    /// A caller-supplied object which will hold the parsed response from DNS.
    /// The response object is (or descends from) isc::dns::Message and is
    /// populated using Message::fromWire().  This method may only be called
    /// once in the lifetime of a Message instance.  Therefore, response_ is a
    /// pointer reference thus allowing this class to replace the object
    /// pointed to with a new Message instance each time a message is
    /// received. This allows a single DNSClientImpl instance to be used for
    /// multiple, sequential IOFetch calls. (@todo Trac# 3286 has been opened
    /// against dns::Message::fromWire.  Should the behavior of fromWire change
    /// the behavior here with could be reexamined).
    D2UpdateMessagePtr& response_;

    /// @brief A caller-supplied external callback which is invoked when DNS
    /// message exchange is complete or interrupted.
    DNSClient::Callback* callback_;

    /// @brief A Transport Layer protocol used to communicate with a DNS.
    DNSClient::Protocol proto_;

    /// @brief TSIG context used to sign outbound and verify inbound messages.
    dns::TSIGContextPtr tsig_context_;

    /// @brief TSIG key name for stats.
    std::string tsig_key_name_;

    /// @brief Constructor.
    ///
    /// @param response_placeholder Message object pointer which will be updated
    /// with dynamically allocated object holding the DNS server's response.
    /// @param callback Pointer to an object implementing @c DNSClient::Callback
    /// class. This object will be called when DNS message exchange completes or
    /// if an error occurs. NULL value disables callback invocation.
    /// @param proto caller's preference regarding Transport layer protocol to
    /// be used by DNS Client to communicate with a server.
    DNSClientImpl(D2UpdateMessagePtr& response_placeholder,
                  DNSClient::Callback* callback,
                  const DNSClient::Protocol proto);

    /// @brief Destructor.
    virtual ~DNSClientImpl();

    /// @brief This internal callback is called when the DNS update message
    /// exchange is complete. It further invokes the external callback provided
    /// by a caller. Before external callback is invoked, an object of the
    /// D2UpdateMessage type, representing a response from the server is set.
    virtual void operator()(asiodns::IOFetch::Result result);

    /// @brief Starts asynchronous DNS Update using TSIG.
    ///
    /// @param io_service IO service to be used to run the message exchange.
    /// @param ns_addr DNS server address.
    /// @param ns_port DNS server port.
    /// @param update A DNS Update message to be sent to the server.
    /// @param wait A timeout (in milliseconds) for the response. If a response
    /// is not received within the timeout, exchange is interrupted. This value
    /// must not exceed maximal value for 'int' data type.
    /// @param tsig_key A pointer to an @c D2TsigKeyPtr object that will
    /// (if not null) be used to sign the DNS Update message and verify the
    /// response.
    void doUpdate(asiolink::IOService& io_service,
                  const asiolink::IOAddress& ns_addr,
                  const uint16_t ns_port,
                  D2UpdateMessage& update,
                  const unsigned int wait,
                  const D2TsigKeyPtr& tsig_key);

    /// @brief This function maps the IO error to the DNSClient error.
    ///
    /// @param result The IOFetch result to be converted to DNSClient status.
    /// @return The DNSClient status corresponding to the IOFetch result.
    DNSClient::Status getStatus(const asiodns::IOFetch::Result result);

    /// @brief This function updates statistics.
    ///
    /// @param stat The statistic name to be incremented.
    /// @param update_key The flag indicating if the key statistics should also
    /// be updated.
    void incrStats(const std::string& stat, bool update_key = true);
};

DNSClientImpl::DNSClientImpl(D2UpdateMessagePtr& response_placeholder,
                             DNSClient::Callback* callback,
                             const DNSClient::Protocol proto)
    : in_buf_(new OutputBuffer(DEFAULT_BUFFER_SIZE)),
      response_(response_placeholder), callback_(callback), proto_(proto) {

    // Response should be an empty pointer. It gets populated by the
    // operator() method.
    if (response_) {
        isc_throw(isc::BadValue, "Response buffer pointer should be null");
    }

    // @todo Currently we only support UDP. The support for TCP is planned for
    // the future release.
    if (proto_ == DNSClient::TCP) {
        isc_throw(isc::NotImplemented, "TCP is currently not supported as a"
                  << " Transport protocol for DNS Updates; please use UDP");
    }

    // Given that we already eliminated the possibility that TCP is used, it
    // would be sufficient  to check that (proto != DNSClient::UDP). But, once
    // support TCP is added the check above will disappear and the extra check
    // will be needed here anyway.
    // Note that cascaded check is used here instead of:
    //   if (proto_ != DNSClient::TCP && proto_ != DNSClient::UDP)..
    // because some versions of GCC compiler complain that check above would
    // be always 'false' due to limited range of enumeration. In fact, it is
    // possible to pass out of range integral value through enum and it should
    // be caught here.
    if (proto_ != DNSClient::TCP) {
        if (proto_ != DNSClient::UDP) {
            isc_throw(isc::NotImplemented, "invalid transport protocol type '"
                      << proto_ << "' specified for DNS Updates");
        }
    }
}

DNSClientImpl::~DNSClientImpl() {
}

void
DNSClientImpl::operator()(asiodns::IOFetch::Result result) {
    // Get the status from IO. If no success, we just call user's callback
    // and pass the status code.
    DNSClient::Status status = getStatus(result);
    if (status == DNSClient::SUCCESS) {
        // Allocate a new response message. (Note that Message::fromWire
        // may only be run once per message, so we need to start fresh
        // each time.)
        response_.reset(new D2UpdateMessage(D2UpdateMessage::INBOUND));

        // Server's response may be corrupted. In such case, fromWire will
        // throw an exception. We want to catch this exception to return
        // appropriate status code to the caller and log this event.
        try {
            response_->fromWire(in_buf_->getData(), in_buf_->getLength(),
                                tsig_context_.get());
            incrStats("update-success");
        } catch (const isc::Exception& ex) {
            status = DNSClient::INVALID_RESPONSE;
            LOG_DEBUG(d2_to_dns_logger, isc::log::DBGLVL_TRACE_DETAIL,
                      DHCP_DDNS_INVALID_RESPONSE).arg(ex.what());
            incrStats("update-error");
        }

        if (tsig_context_) {
            // Context is a one-shot deal, get rid of it.
            tsig_context_.reset();
        }
    } else if (status == DNSClient::TIMEOUT) {
        incrStats("update-timeout");
    } else {
        incrStats("update-error");
    }

    // Once we are done with internal business, let's call a callback supplied
    // by a caller.
    if (callback_ != NULL) {
        (*callback_)(status);
    }
}

DNSClient::Status
DNSClientImpl::getStatus(const asiodns::IOFetch::Result result) {
    switch (result) {
    case IOFetch::SUCCESS:
        return (DNSClient::SUCCESS);

    case IOFetch::TIME_OUT:
        return (DNSClient::TIMEOUT);

    case IOFetch::STOPPED:
        return (DNSClient::IO_STOPPED);

    default:
        ;
    }
    return (DNSClient::OTHER);
}

void
DNSClientImpl::doUpdate(asiolink::IOService& io_service,
                        const IOAddress& ns_addr,
                        const uint16_t ns_port,
                        D2UpdateMessage& update,
                        const unsigned int wait,
                        const D2TsigKeyPtr& tsig_key) {
    // The underlying implementation which we use to send DNS Updates uses
    // signed integers for timeout. If we want to avoid overflows we need to
    // respect this limitation here.
    if (wait > DNSClient::getMaxTimeout()) {
        isc_throw(isc::BadValue, "A timeout value for DNS Update request must"
                  " not exceed " << DNSClient::getMaxTimeout()
                  << ". Provided timeout value is '" << wait << "'");
    }

    // Create a TSIG context if we have a key, otherwise clear the context
    // pointer.  Message marshalling uses non-null context is the indicator
    // that TSIG should be used.
    if (tsig_key) {
        tsig_context_ = tsig_key->createContext();
        tsig_key_name_ = tsig_key->getKeyName().toText();
    } else {
        tsig_context_.reset();
        tsig_key_name_.clear();
    }

    // A renderer is used by the toWire function which creates the on-wire data
    // from the DNS Update message. A renderer has its internal buffer where it
    // renders data by default. However, this buffer can't be directly accessed.
    // Fortunately, the renderer's API accepts user-supplied buffers. So, let's
    // create our own buffer and pass it to the renderer so as the message is
    // rendered to this buffer. Finally, we pass this buffer to IOFetch.
    dns::MessageRenderer renderer;
    OutputBufferPtr msg_buf(new OutputBuffer(DEFAULT_BUFFER_SIZE));
    renderer.setBuffer(msg_buf.get());

    // Render DNS Update message. This may throw a bunch of exceptions if
    // invalid message object is given.
    update.toWire(renderer, tsig_context_.get());

    // IOFetch has all the mechanisms that we need to perform asynchronous
    // communication with the DNS server. The last but one argument points to
    // this object as a completion callback for the message exchange. As a
    // result operator()(Status) will be called.

    // Timeout value is explicitly cast to the int type to avoid warnings about
    // overflows when doing implicit cast. It should have been checked by the
    // caller that the unsigned timeout value will fit into int.
    IOFetch io_fetch(IOFetch::UDP, io_service, msg_buf, ns_addr, ns_port,
                     in_buf_, this, static_cast<int>(wait));

    // Post the task to the task queue in the IO service. Caller will actually
    // run these tasks by executing IOService::run.
    io_service.post(io_fetch);

    // Update sent statistics.
    incrStats("update-sent");
    if (tsig_key) {
        incrStats("update-signed", false);
    } else {
        incrStats("update-unsigned", false);
    }
}

void
DNSClientImpl::incrStats(const std::string& stat, bool update_key) {
    StatsMgr& mgr = StatsMgr::instance();
    mgr.addValue(stat, static_cast<int64_t>(1));
    if (update_key && !tsig_key_name_.empty()) {
        mgr.addValue(StatsMgr::generateName("key", tsig_key_name_, stat),
                     static_cast<int64_t>(1));
    }
}

DNSClient::DNSClient(D2UpdateMessagePtr& response_placeholder,
                     Callback* callback, const DNSClient::Protocol proto)
    : impl_(new DNSClientImpl(response_placeholder, callback, proto)) {
}

DNSClient::~DNSClient() {
}

unsigned int
DNSClient::getMaxTimeout() {
    static const unsigned int max_timeout = std::numeric_limits<int>::max();
    return (max_timeout);
}

void
DNSClient::doUpdate(asiolink::IOService& io_service,
                    const IOAddress& ns_addr,
                    const uint16_t ns_port,
                    D2UpdateMessage& update,
                    const unsigned int wait,
                    const D2TsigKeyPtr& tsig_key) {
    impl_->doUpdate(io_service, ns_addr, ns_port, update, wait, tsig_key);
}

} // namespace d2
} // namespace isc